The build-up of a printed circuit board for electronic and computer equipment entails the connection of electronic devices, sub-assemblies, and other such components to the board. A common means for building up printed circuit boards, sometimes referred to as "through-hole" technology, involves mounting components on one side of a board by inserting their leads through holes extending through the board. The components are affixed to the board by soldering the lead ends where they protude from the back side of the board. A conventional soldering technique is "wave soldering" where a printed circuit board with loosely held components is passed over a bath of molten solder. The lead ends are soldered as they are contacted by a wave of the molten solder.
An emerging technology for the build up of printed circuit boards is referred to as "surface mount" technology, and involves the mounting of components on both sides of a printed circuit board. Surface mount technology has one obvious advantage over through-hole technology in the greater number of components that can be mounted per board. However, the highly automated technique of wave soldering is unusable. Individual components may be mounted to the board surface by hand soldering. This is both time consuming and expensive. A preferred process for mass production involves heating the board and components. Both hot air and infrared (IR) radiation are used for such heating. For example, an IR reflow oven heats the various components, using infrared radiation, until the solder melts (about 218.degree. C).
The relatively high temperature of the IR reflow oven presents some problems. Many of the electronic devices to be mounted on the board are so-called "plastic packaged devices". Plastic packaged devices are formed by encapsulating a semiconductor chip in a plastic package. These plastic packaged devices typically have a plurality of thin, gold bonding wires which connect the contacts of the chip with the external leads of the device. Both the chip and wire are encapsulated by the plastic with only the external leads extending from the device. The plastic, typically an epoxy resin, has the property of absoring moisture when subject to a hunid environment. When subjected to the high temperature of the IR reflow oven, any moisture in the plastic is rapidy vaporized. The vapor expansion may create small cracks which propagate through the plastic. These cracks appear as cosmetic flaws in the chip which can create later problems as water enters the crack, penetrates to the chip and corrodes the contacts. The cracks can also create a more immediate problem by breaking the thin bonding wires. This cracking problem is particularly acute in larger devices such as relatively high pin count chips. Sometimes the chip itself may have microscopic cracks in it caused by excessive pressure applied between the chip contact and bonding wires during manufacture (known as overbonding). These microscopic cracks may casue no harm. However, moisture may collect in the cracks and when rapidly vaporized during soldering crack the chip itself (known as chip-out).
The conventional way of avoiding the cracking problem is to bake the plastic packaged devices at a lower temperature (about 125.degree. C. for 24 hours) prior to processing in the IR reflow oven. This allows the moisture to be vaporized more slowly permitting the vapor to released from the plastic without cracking the device. Several problems exist with this baking process. First, the external leads of an electronic device are normally dipped or coated with a tin/lead solder. The solder will not melt below about 183.degree. C.; however, heating the solder above about 60.degree. C. will start to oxidize the solder. The soldered leads can be heated above 60.degree. C. one time prior to processing in the IR reflow oven. However, once so heated the solder cannot be reheated above 60.degree. C. a second time prior to processing without the lead/tin solder oxidizing. Since oxidized leads render chips unusable, once a device is baked to drive off absorbed moisture it must be protected from further humidity until processed in the IR reflow oven. Devices that are not to be mounted for IR reflow oven processing within 96 hours are normally sealed in vapor tight bags with a desiccant. The cost of preventing the devices from reabsoring moisture after being baked is undesirable.
Another problem with the baking process involves the various types of protective carriers used to transport and store electronic devices. For example, plastic tubes are used to prevent mechanical damage to the device leads, various static shielding containers are used to prevent electrical damage to the device, and tape and reel configurations separate and orient smaller devices for automated assembly. In most cases, removing the devices from their carriers for the baking process defeats the purpose of the carrier, e.g., leads get bent, chips get blown with electrostatic charges and the organization of taped components is lost. However, in most cases 125.degree. C. exceeds the critical temperature at which the carrier structure degrades, i.e., the temperature at which plastic tubes, static shields and plastic carrier tape melt or otherwise deteriorate. Thus, in order to bake devices to drive off moisture, they must be removed from their protective carriers which inevitably leads to additional part losses and other expenses. Conventional processing techniques, such as described above, have in the past resulted in losses as high as 42% for high pin count devices.